Alkaline batteries, such as nickel-hydride batteries and nickel-cadmium batteries, generally consist of a power-generating assembly housed in a battery case that serves as one of the electrode terminals. For example, as shown in FIG. 10, there is proposed a current collector that includes a current collecting plate 101 and a current collecting lead plate 103, which have the same thickness and are formed integrally with each other.
As shown in FIG. 11, this type of battery has a power-generating assembly in which a positive electrode plate 8 and a negative electrode plate 9 are spirally wound with a separator 10 disposed therebetween. The power-generating assembly is housed in a metallic battery case that serves as an outer container 6. A sealing assembly 11, welded at one point to a current collecting lead plate 103, is mounted at the opening of the battery case 6 to seal the opening. An insulation gasket is arranged between the sealing assembly 11 and the battery case 6.
When this type of alkaline battery is used in electric power tools, electric automobiles and other applications that repeat charge and discharge at a high rate, the electrical resistance of the current collector that connects between the power-generating assembly and the sealing assembly significantly affects the battery performance. Since these applications often require charging and discharging at high currents, the internal resistance of the battery must be minimized.
Batteries with decreased internal resistance are known as below: (See, for example, Patent Document 1) Patent Document 1 JP-A-2004-63272 (FIGS. 1 through 4, 10 and 11; paragraphs [0022] through [0038])
A nickel-cadmium battery based on the low internal resistance battery described in Patent Document 1 is now described.
FIG. 12 is a perspective view showing main components of a nickel-cadmium battery having a current collector integrally formed by blanking. FIGS. 13(a) and 13(b) are a plan view and a cross-sectional view of the current collector 1, respectively. The current collector is formed of a 0.3 mm-thick nickel-plated steel plate and includes a flat portion 2 and a projection 3 that is projected about 2.0 mm in height and formed by blanking.
The current collector is a substantially disk-shaped component with the top surface of the projection 3 being formed as a thin area 4 that serves as a weld area.
The flat portion includes bores 5 formed therein. Formed on the edge of each bore is a burr 5B that projects on the backside of the current collector and forms a weld point for welding to the positive electrode plate. FIG. 14 is a cross-sectional view showing the manner in which an electrode body is inserted into a battery case 6 which serves as an outer container and welded to a sealing assembly via the current collector 1.
As shown in FIG. 14, the nickel-cadmium battery includes a battery assembly in which a nickel positive electrode plate 8 and a cadmium negative electrode plate 9 are spirally wound with a separator 10 disposed therebetween. The battery assembly is housed in a battery case 6, which is a bottomed cylinder made of nickel-plated steel. The above-described current collector 1 is mounted on top of the battery assembly and a sealing assembly 11 is welded to the projection 3 of the current collector 1 by direct welding.
The sealing assembly 11 consists of a lid 12 having a circular projection projecting downward from the bottom thereof, a positive electrode cap 13, and a valve body disposed between the lid 12 and the positive electrode cap 13 and consisting of a spring 15 and a valve plate 14. A vent 16 is formed at the center of the lid.
As described, the edge of each of the bores 5 formed in the flat portion 2 is formed as the burr 5B that projects on the backside of the current collector 1 and forms a weld point for welding to the positive electrode plate 8. Before welding of the sealing assembly to the current collector 1, the nickel positive electrode plate is welded to the current collector 1 via these weld points. Disposed at the bottom of the battery case 6 is a disk-shaped negative electrode current collector 7 that is welded to the negative electrode plate 9. The opening part 17 of the battery case 6 is sealed by caulking.
According to the above-described construction, effective weld areas can be readily obtained simply by forming a single circular metal plate by punching out. As a result, firm and reliable connection can be achieved.
In addition, the connection resistance can be reduced since the flat portion 2, which serves as the current collecting body to be connected to the electrode, and the projection 3, which serves as the current collecting lead to be connected to the positive electrode terminal (sealing assembly), can be formed integrally.
As shown in FIG. 13(b), the welding current is concentrated to the thin top surface 4 of the projection 3. Also, the thin top surface has an elasticity that ensures that the weld area is firmly held under pressure. Thus, this construction further ensures firm connection.
Although the weld area can be firmly held under pressure according to the above-described method for manufacturing batteries, the pressure may vary significantly (for example, excessive pressure may apply) and the welding current may become insufficient, resulting in the formation of defective welds that are difficult to detect.
Other types of low internal resistance batteries are also known (See, for example, Patent Documents 2 and 3).
Patent Document 2: JP-A-2001-143684 (FIGS. 1, 14 and 15. As well as FIGS. 15, 16 and 17 of the drawings accompanying the present application)
Patent Document 3: JP-A-2001-155710 (FIGS. 3 and 4. As well as FIGS. 18 and 19 of the drawings accompanying the present application)
A low internal resistance battery described in Patent Document 2 has a structure shown in FIGS. 15, 16 and 17. The battery includes a cylindrical lead. One variation of this battery employs welding techniques that are described as follows: “The main body 21 of the cylindrical body 20 is arranged along the diameter of the positive electrode current collector 14 and wing-like portions 22, 22 are arranged on the positive electrode current collector 14. The wing-like portions 22, 22 are then welded to the positive electrode current collector 14 by spot-welding (First welding). “(paragraph [0053]) “A press is operated to descend a punch P and press the seal portion of the sealing assembly 17 (the edge 16b of the open end of the battery case 16). At the same time, a voltage of 24V is applied between a pair of welding electrodes W1 and W2 in the direction of battery discharge to cause a 3 kA current to flow for about 15 msec while the welding electrodes W1 and W2 are being pressed at a pressure of 2×106 N/m2 (Second welding). In this manner, the sealing assembly 17 is pushed into the battery case 16, and the bottom surface of the sealing assembly 17 and the periphery of the main body 21 of the cylindrical body 20 are welded together where they contact, forming welds.” (paragraph [0056])
However, the second welding in manufacturing a battery requires a large welding current in order to weld the lead to the thick lid. If the welding current is excessively large, the resulting heat may cause the points of contact on the lead to meltdown. As a result, the close contact at the welds can no longer be maintained and the firmness of welding may be reduced, resulting in the formation of significantly varied welds. In addition, defective welds are often produced since the conditions for the welding are not properly determined. Such defective welds are difficult to detect.
Patent Document 2 also describes that “While in the foregoing embodiments and their variations, the cylindrical bodies 20, 30, 40, 50, 60 and 70 are first welded to the positive electrode current collector 14 and then welded to the sealing assembly 17 by passing a current between the positive electrode external terminal (positive electrode cap) 17a and the negative electrode external terminal (the bottom surface of the battery case 16), the cylindrical bodies 20, 30, 40, 50, 60 and 70 may first be welded to the sealing assembly 17 and subsequently welded to the positive electrode current collector 14 by passing a current between the positive electrode external terminal (positive electrode cap) 17a and the negative electrode external terminal (the bottom surface of the battery case 16). This gives the same results.” (paragraph [0060]) However, Patent Document 2 provides no description as to what conditions are required to weld the sealing assembly (lid) to the cylindrical body (lead) first and then weld the positive electrode connector (upper current collector) to the lead.
Shown in FIGS. 18 and 19 is another low internal resistance battery described in Patent Document 3. As shown, the battery includes a battery case 16 that also serves as one of the electrode terminals and has an opening, a sealing assembly 17 (lid 17a, positive electrode cap 17b, spring 17c and valve body 17d) that also serves as the other of the electrode terminals and seals the opening of the battery case 16, a positive electrode plate 11 housed in the battery case 16, and an electrode body 10 having a negative electrode plate 12 at least one end of which is connected to a current collector 14. The sealing assembly 17 is connected and secured to the current collector 14 via a lead member, which consists of a rim-shaped cylindrical body 20 having a narrow central portion as viewed in the longitudinal direction. The rim-shaped cylindrical body 20 includes at the upper and lower ends thereof flanges 22, 23 that consist of wide portions 22a, 23a and narrow portions 22b, 23b formed alternately. The wide portions 22a are at a distance from the corresponding narrow portions 23b in an overlapping arrangement, whereas the narrow portions 22b are at a distance from the corresponding wide portions 23a in an overlapping arrangement.
A nickel-hydride rechargeable battery having the lead member consisting of the rim-shaped cylindrical body 20 is manufactured by using a welding technique according to the following description.
To assemble the nickel-hydride rechargeable battery, the above-described rim-shaped cylindrical body 20 was first placed on the positive electrode current collector 14. With welding electrodes (not shown) positioned at the outer periphery of the narrow portions 22b of the upper flange, the wide portions 23a of the lower flange were welded to the current collector 14 by spot-welding. Subsequently, the electrode body 10, consisting of the rim-shaped cylindrical body 20 welded to the positive electrode current collector 14, was inserted into the bottomed cylindrical battery case 16 made of nickel-plated steel (the outer surface of the bottom serves as the negative electrode external terminal). (paragraph [0025])
After the sealing assembly 17 was arranged as described above, one welding electrode W1 was arranged at the top surface of the positive electrode cap (positive electrode external terminal) 17a and the other welding electrode W2 was arranged at the lower side of the bottom of the battery case 16 (negative electrode external terminal). A voltage of 24V was then applied between the pair of welding electrodes W1 and W2 in the direction of battery discharge to cause a 3 kA current to flow for about 15 msec while the welding electrodes W1 and W2 were being pressed toward each other at a pressure of 2×106 N/m2. In this manner, the current was concentrated to points of contact between the bottom surface of the sealing assembly 17 and the small projections 22c formed in the wide portions 22a of the upper flange 22 of the rim-shaped cylindrical body 20, so that the small projections 22c are welded to the bottom surface of the sealing assembly 17, forming welds. At the same time, the bottom surface of the negative electrode current collector 15 and the upper side of the bottom of the battery case 16 (negative electrode external terminal) were welded together where they contact, forming welds. (paragraph [0027])
Subsequently, an insulation gasket 19 was fitted to the periphery of the sealing assembly 17. Using a press, the sealing assembly 17 was pushed into the battery case 16 until the lower end of the insulation gasket 19 was positioned at the recess 16a. The edge of the open end of the battery case 16 was then caulked inward to seal the battery. This completed a cylindrical nickel-hydride rechargeable battery with a nominal capacity of 6.5 Ah. The pressure applied during the sealing process collapsed the main body 21 of the rim-shaped cylindrical body 20 at the narrow central portion. (paragraph [0028])
Another method for manufacturing a cylindrical nickel-hydride rechargeable battery with a nominal capacity of 6.5 Ah by welding before and after the sealing is also described as follows:
The above-described rim-shaped cylindrical body 20 was first placed on the positive electrode current collector 14. With welding electrodes (not shown) positioned at the outer periphery of the narrow portions 22b of the upper flange 22, the wide portions 23a of the lower flange 23 were welded to the current collector 14 by spot-welding. Subsequently, the electrode body 10, consisting of the rim-shaped cylindrical body 20 welded to the positive electrode current collector 14, was inserted into the bottomed cylindrical battery case 16 made of nickel-plated steel (the outer surface of the bottom serves as the negative electrode external terminal).
(paragraph [0029])
Subsequently, an insulation gasket 19 was fitted about the periphery of the sealing assembly 17. Using a press, the sealing assembly 17 was pushed into the battery case 16 until the lower end of the insulation gasket 19 was positioned at the recess 16a. The edge of the open end of the battery case 16 was then caulked inward to seal the battery. The pressure applied during the sealing process collapsed the main body 21 of the rim-shaped cylindrical body 20 at the narrow central portion. One of the welding electrode W1 was arranged at the top surface of the positive electrode cap (positive electrode external terminal) 17a and the other of the welding electrodes W2 was arranged at the lower side of the bottom of the battery case 16 (negative electrode external terminal). (paragraph [0031])
A voltage of 24 V was then applied between the pair of welding electrodes W1 and W2 in the direction of battery discharge to cause a 3 kA current to flow for about 15 msec while the welding electrodes W1 and W2 were being pressed toward each other at a pressure of 2×106 N/m2. In this manner, the current was concentrated to points of contact between the bottom surface of the sealing assembly 17 and the small projections 22c formed in the wide portions 22a of the upper flange 22 of the rim-shaped cylindrical body 20, so that the small projections 22c are welded to the bottom surface of the sealing assembly 17, forming welds. At the same time, the bottom surface of the negative electrode current collector 15 and the upper side of the bottom of the battery case 16 (negative electrode external terminal) were welded together where they contact, forming welds. (paragraph [0032])
However, the resulting battery has a problem that, when a large welding current is applied in order to weld the rim-shaped cylindrical body (lead) to the thick sealing assembly (lid), the welds of the positive electrode current collector (upper current collector) may break because of the large current, resulting in a decreased firmness of welding and a significant variation in the resistance of the lead. In addition, defective welds often produced since the conditions for the welding are not properly determined. Such defective welds are difficult to detect.
Batteries employing short conductive paths to reduce the internal resistance are also known (See, for example, Patent Documents 4 through 6).
Patent Document 4: JP-A-2004-259624 (FIG. 1. As well as FIG. 20 of the drawings accompanying the present application.)
Patent Document 5: JP-A-2004-235036 (FIGS. 6, 14 and 15. As well as FIGS. 21, 22 and 23 of the drawings accompanying the present application.)
Patent Document 6: JP-A-10-261397 (FIG. 1. As well as FIG. 24 of the drawings accompanying the present application.)
In batteries described in Patent Documents 4 through 6, a current collecting lead is welded between a terminal and an electrode. After sealing, the area that is to be caulked is press-fitted by pushing with a press. This brings the projections formed on the current collecting lead into contact with the opposing surface, thus completing a short conductive path. As a result, the current collecting resistance can be reduced.
Although the current collecting lead forms a short conductive path within the internal space as it is pressed and deformed, some of the current flows through other paths as reactive current upon welding of the current collecting lead to the contacts that form the short conductive paths. This makes it difficult to achieve firm welding and results in a significant variation in the resistance.
Furthermore, an oxide film may be formed on the short conductive path depending on the conditions under which the battery is used since the current collecting lead is exposed to the electrical potential of the positive electrode. As a result, the resistance may gradually increase during the use of the battery.
In a battery described in Patent Document 5, the short conductive path is welded before the battery is sealed by caulking the edge 16b of the open end of the outer container 16 inwardly. Thus, the resulting conductive path is not short enough to significantly reduce the resistance.
A battery described in Patent Document 6 is manufactured by a method including the following steps: sealing the opening of the battery case with the sealing assembly; and subsequently passing a current between the battery case and the sealing assembly to weld points of contact between the current collecting lead plate and the sealing assembly and thus form welds. This technique makes it possible to easily mount the sealing assembly to the opening of outer container even when the current collecting lead is short and to reduce the internal resistance of the battery by decreasing the current collecting distance. In addition, the technique eliminates the need to bend the current collecting lead upon sealing of the battery and thus allows the use of thick current collecting leads. This also facilitates the reduction of the internal resistance of the battery.
In the above-described technique, however, part of the current collecting lead plate leading from one of the positive and negative electrodes is brought into contact with the bottom surface of the sealing assembly and, subsequently, the current collecting lead plate and the sealing assembly are welded together where they contact, forming welds. The technique therefore cannot achieve firm welding. Also, the contact between the sealing assembly and current collecting lead cannot be ensured, nor can firm welds be formed by this technique when the height of the electrode body housed in the outer container varies since the current collecting structure described in examples cannot properly absorb the variation in the height of electrode body.